More on the Trump campaign

His chief rival is Texas Senator Ted Cruz, but one of his other rivals, the big-spending former Florida Governor Jeb Bush, will be hoping for a better performance after disappointment in Iowa and New Hampshire.

On Tuesday, a tweet by Mr Bush depicting a gun engraved with his name, alongside the word "America", provoked a strong reaction online.

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Within hours it had been retweeted 15,000 times and was met by outrage and parody.

US fugitive Edward Snowden told Mr Bush to delete his account.

And Piers Morgan, who used to host a primetime show on CNN and was well known for his stance against gun rights, said it was "appalling".

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Mr Bush tweeted the picture after taking questions at the premises of gun maker FN Manufacturing in Columbia, South Carolina.

The Washington Post said the tweet "has the potential to boost his image among Southerners beyond South Carolina who value the 2nd Amendment and the right to bear arms".

The Democratic race will focus on Saturday on Nevada, where Hillary Clinton and Bernie Sanders will do battle in a caucus, which operates as a public show of support rather than a closed ballot.